Making a Voronoi Texture affect where an image texture is placed on a model?
You can add a Color > Hue Saturation Value node and plug the voronoi texture into the Saturation socket.  Values on the voronoi texture closer to 1 will make those areas of the diffuse texture closer to the original saturation, values closer to 0 will be more saturated.



You can add a Color Ramp node after the Voronoi texture to have better control.